How do I get a Roblox Stormtrooper helmet to not be greyed out?
There are a few reasons why this could be happening.
Listed as follows are some, with their solutions.

You are using EEVEE renderer instead of Cycles.
Change this by going to the Render properties tab, and setting the renderer to Cycles.

You don't have a material set.
Go to the material tab in the object properties with the object you want a material set for selected, and add/adjust your material.


You need to adjust the material set for your object to something other than the default.
In the images, I try to achieve a material somewhat like a stormtrooper helmet.
First adjust the roughness, then adjust the metallic-ness of the Principled BSDF.

I believe you are doing this already, but it is always good to *Compare your object material to another object in the scene, with Rendered view selected.
In these photos I compare the cube object with the helmet material to a blue Suzanne &quot;monkey&quot; head. (Also with a principled shader.)


With no shader adjustment:
With roughness set to 0:
With final material:
One last tip: Adjusting world shader can sometimes give you a better preview of your materials. Here are examples of world brightness and color with a scene of objects with the helmet shader:
Sometimes a dark world background with a bright light helps too:
I hope this helps!